Well bores or boreholes are drilled using a drilling assembly (also referred to as the "bottom hole assembly" or "BHA") carrying a drillbit at its bottom hole end. The BHA includes a variety of sensors to gather information about the wellbore and subsurface formations along with associated processing circuits and microprocessors. Data and signals are transmitted from the surface to control the operation of devices in the BHA. Such devices include motors, hydraulic devices, etc. A number of signal transmission methods have been used to send signals from the surface to a receiver in the BHA. In one such method, an acoustic signal carried by the mud or by the drillstring is used. Electromagnetic signals carried by the drillstring have also been used to transmit information downhole. However, these methods are difficult to use in measurement-while-drilling ("MWD") operations because of the necessity of maintaining an adequate mud flow for drilling operations and of the noise associated with the mud flow and with the rotating drillstring. A common method of communicating the signals downhole is via drilling fluid pressure pulses ("mud pulses") generated by altering the rate of flow of the drilling mud used in drilling operations.
This is fraught with problems because of the wear and tear on the mud pumps from constant starting and stopping. A major accompanying problem is that the mud pulses attenuate and disperse as they propagate through the drilling mud. This dispersion is unavoidable and is caused by various mechanisms, including viscous dissipation in the drilling mud as well as frictional energy loss at the borehole walls. This problem is exacerbated with increasing depth of the wellbore. When a square wave is transmitted through a dispersive medium, the received signal is no longer a square wave; instead of a sharp change in amplitude corresponding to the leading and trailing edges of the square wave, the received signal shows a gradual change in amplitude. In addition, the received signal is attenuated compared to the transmitted signal.
Because of the dispersion and attenuation of the signal, detection of the onset of the pulses and the determination of their duration can be difficult. Without proper decoding of the pulses, control of the downhole equipment is lost. As noted above, the problem gets worse as drilling depth increases due to increased attenuation and dispersion. The ability to detect pulses determines the bandwidth of the mud pulse telemetry link. Prior art techniques have relied on an ad hoc method of dealing with the problem: the pulse duration is increased by predetermined increments as the drilling depth increases. As an example, a pulse duration of 8 seconds is used at shallow depths, of 12 seconds at intermediate depths and of 16 seconds at large depths. This is an inefficient procedure for as it does not allow for maximum data transmission based on the available bandwidth of the data channel. Furthermore, the limited choice of available pulse duration means that if the predetermined discrete values are inadequate, the entire drillstring has to be brought to the surface to adjust the downhole tool for another data rate.
It is desirable to have a method and an apparatus for adjusting the pulse telemetry that automatically adjusts for the attenuation of the signal by adjusting the data rate. The method should preferably make use of the full available bandwidth for signal transmission. It should also not require retrieval of the downhole equipment to modify the data transmission rate. The present invention provides a downhole telemetry system that automatically adjusts the data rate as a function of the deterioration of the transmitted signal during drilling of the wellbore.